The present invention relates generally to methods of completing subterranean wells, and, in a preferred embodiment thereof, more particularly provides a method which facilitates the placement of sand control screens and perforating guns opposite formations in the wells.
In the course of completing an oil and/or gas well, it is common practice to run a string of protective casing into the wellbore and then to run production tubing inside the casing. At the wellsite, the casing is perforated across one or more production zones to allow production fluids to enter the casing bore. During production of the formation fluid, formation sand is also swept into the flow path. The formation sand is typically relatively fine sand that tends to erode production equipment in the flow path.
One or more sand screens are typically installed in the flow path between the production tubing and the perforated casing. A packer is customarily set above the sand screen to seal off the annulus in the zone where production fluids flow into the production tubing. In the past, it was usual practice to install the sand screens in the well after the well had been perforated and the guns either removed from the wellbore or dropped to the bottom of the well.
Well completion methods continue to utilize time and resources more efficiently by running the guns, sand screens, and packer into the well on the production tubing in only one trip into the well. From the end of the production tubing down, the completion tool string typically consists of a releasable packer (one capable of being set, released, and reset in the casing, whether by mechanical or hydraulic means), sand control screens, and perforating guns. The completion string is lowered into the well until the guns are opposite the formation to be produced, the packer is set to seal off the annulus above the packer from the formation to be produced, the guns are fired to perforate the casing, the packer is unset, the completion string is again lowered until the sand screens are opposite the perforated casing, the packer is reset, and the formation fluids are then produced from the formation, through the sand screens, into the production tubing, and thence to the surface.
This method has several disadvantages, however. One disadvantage is that a significant amount of rig time is consumed while unsetting, repositioning, and resetting the packer. The rig operator must typically lift the production tubing, manipulate the tubing to unset the packer, lower the tubing into the well a predetermined distance, manipulate the tubing to set the packer, apply tubing weight to the packer, and, finally, perform tests to determine whether the packer has been properly set.
Another disadvantage of the method is that the above-described packer unsetting, repositioning, and resetting must be performed after the casing has been perforated. A necessary consequence of this situation is the possibility that formation fluids may enter the wellbore, and in an extreme situation may even cause loss of control of the well. For this reason, during the packer unsetting, repositioning, and resetting, the well is overbalanced at the formation during these operations--meaning that the pressure in the wellbore is maintained at a level greater than the pressure in the formation. This, in turn, means that wellbore fluids enter the formation through the perforations in the casing, possibly causing damage to the formation.
Furthermore, the method suffers from problems encountered when attempting to reset a packer. In general, modern releasable packers are fairly reliable when lowered into a wellbore and set in casing at a particular location. When, however, a releasable packer is set and then unset and moved to another location, its reliability is greatly diminished. The slips (which grip the interior wall of the casing) may no longer hold fast, and the packer rubbers (which seal against the casing) may not seal adequately a second time.
Additionally, there are other circumstances where, in the drilling, completion, rework, etc. of a well, it is necessary to reposition equipment in the well. Frequently, in these circumstances, it is inconvenient to reposition the equipment by manipulating tubing at the surface, repositioning a packer, or by other methods heretofore known. As an example, in modern practice it is common to run more than one set of perforating guns into a well in one trip. The guns are typically spaced apart with tubing such that each set of guns is positioned opposite a separate formation or pay zone before the guns are fired. If the guns could be repositioned after a first set of guns were fired into a formation, so that a subsequent set of guns would be positioned opposite another formation, the tubing used to space apart the guns could be eliminated and the production string could be shortened.
From the foregoing, it can be seen that it would be quite desirable to provide well completion methods which do not require repositioning a releasable packer, but which permit sand control screens to be run into the well with perforating guns in one trip and then position the sand control screens opposite the formation after the casing has been perforated. It is accordingly an object of the present invention to provide such well completion methods.
In addition, it is desirable to provide methods for positioning other equipment in a wellbore. It is accordingly another object of the present invention to provide such methods of positioning equipment in a wellbore.